Mean arterial blood pressure (MABP) in conscious dogs oscillates with a period of 1-3 hours; the causes and consequences of this oscillation will be investigated. Suitably filtered MABP signals from 2-3 days of continuous recording will be analyzed with fast Fourier transform based time series analytic methods to estimate power spectral density so as to determine the frequency with greater precision than has been possible heretofore. Complex demodulation techniques will be used to estimate the interaction of this rhythm with the diurnal rhythm. A possible neural origin of the rhythm will be tested by surgical interruption of cardiac sympathetic efferents, and by correlating EEG and EMG activity with the MABP oscillation; correlation procedures will involve the usual coherence, and phase angle estimates. Since arterial blood pressure is an input to both renin and arginine vasopressin secretory mechanisms, circulating levels of these hormones will be measured to test the hypothesis that the hormones oscillate in plasma. To investigate how the pressure regulator uses the components of the system to generate the oscillation, dogs will be exercised, saline loaded, or hemorrhaged; the relaxation from these perturbations to the oscillatory steady state will then be followed.